Cancer is now the leading cause of death in Japan, and more than 300 thousand people die of cancer every year. Under such circumstances, a particle beam therapy method using a heavy particle beam such as a carbon beam, a proton beam has been attracting attention because of its superior features such as a high therapy effect, fewer side effects, or less physical burden. With this therapy method, a particle beam emitted from an accelerator is applied to cancer cells to kill the cancer cells with a little influence on normal tissues.
A currently used particle beam irradiation method in this therapy method is referred to as an expanded beam method. In the expanded beam method, a beam diameter of a particle beam is expanded to the size of an affected region or more by a method referred to as a wobbler method or a double scatterer method. Then, a brass collimator referred to as a shape collimator is used to limit an irradiation region to substantially match a beam shape with the shape of the affected region. In a beam traveling direction (beam axis direction), the beam is expanded by a beam range expansion device referred to as a ridge filter, and a beam range shaping device made of polyethylene referred to as a bolus matches a beam stop position with the shape (outline) of the affected region in a deep position for irradiation.
However, the expanded beam method cannot strictly three-dimensionally match the beam with the shape of the affected region, and there is a limit in reducing influence on normal tissues around the affected region. In addition, the shape collimator and the bolus are produced for each affected region (and further, each irradiation direction of the affected region), and thus remain as radiation waste after therapy irradiation.
Then, as a further advanced irradiation method of particle beam therapy, a three-dimensional irradiation method of three-dimensionally irradiating an in-body affected region to target cancer cells with higher accuracy has been developed (see Patent Document 1: Japanese Patent Laid-Open No. 2001-212253; Non-patent Document 2: Yasuyuki Futami, and eight others; “Broad-beam three-dimensional irradiation system for heavy-ion radiotherapy at HIMAC”; Nuclear Instruments and Methods in Physics Research A 430 (1999) 143-153; 19, Jan. 1999, or the like).
One of three-dimensional irradiation methods is a method referred to as a scanning irradiation method. This method virtually divides a therapy area into a three-dimensional lattice points and performs irradiation of each lattice point. With such a three-dimensional irradiation method, a beam can be matched with an affected region also in a beam axis direction with high accuracy without using a shape collimator or a bolus, and exposure of normal tissues can be prevented as compared with a conventional two-dimensional irradiation method.
However, the scanning irradiation method has a problem described below. Conventionally, in particle beam therapy, for an organ whose area is moved with respiration such as lung or liver, a respiration waveform signal is obtained, and in-gate irradiation is performed that is irradiation performed only when the area is within a certain position range. However, in the scanning irradiation method, irradiation points are sequentially switched to cause relative displacement of the irradiation points with the movement of the area by respiration, causing non-uniform dose distribution in an irradiation region. To solve this problem, Non-patent Document 1 proposes a respiration synchronized irradiation method described below. In this method, one irradiation time (a time for one irradiation of the entire irradiation region in a slice to be irradiated) in one slice (a planar divided unit of an affected region divided in a beam axis direction) is set to be 1/m of a gate width of one respiration. Then, scanning irradiation of the same slice is repeated m times (for example, m=eight times) in one respiration. After the m time scanning irradiation of the target slice is finished, a slice to be irradiated is changed, and m time scanning irradiation of a next slice to be irradiated is performed in the same manner. In this method, the gate width of the one respiration is divided into m parts, scanning start times for the same slice are dispersed in the gate width of the one respiration (referred to as phase control in Non-patent Document 1), and irradiation of the same slice is repeated m times (referred to as rescanning in Non-patent Document 1. Non-patent Document 1: Takuji Furukawa, and eight others, “Design review of three-dimensional scanning irradiation apparatus”, HIMAC report of National Institute of Radiological Sciences: HIMAC-124, issued by Independent administrative institute, National Institute of Radiological Sciences, April, 2007). As a result, even if the irradiation region is moved in the gate width of one respiration, changes in dose for the irradiation region due to the movement can be reduced by an integral effect, and dose uniformity can be improved according to a statistical error of 1/√{square root over (m)}.
However, the phase control rescanning irradiation proposed in Non-patent Document 1 has a problem described below. Generally, the size of an affected region of a patient is not uniform in a beam axis direction. Specifically, each slice has a different area, and each slice has a different number of lattice points.
In contrast, the gate width of one respiration does not significantly change though there are some changes. However, a scanning time for one of the m divided parts of the gate width of one respiration needs to be the same for every slice as long as the number m for one slice is fixed. This implies that an irradiation time for one lattice point is reduced for a slice with many lattice points (slice with a large area), while an irradiation time for one lattice point is increased for a slice with a few lattice points (slice with a small area).
On the other hand, the dose is determined by a product of an irradiation time and beam intensity. Thus, for example, when the same dose is required for each slice in therapy planning, high beam intensity needs to be set for the slice with many lattice points (slice with a large area), while low beam intensity needs to be set for the slice with a few lattice points (slice with a small area). Specifically, the beam intensity needs to be changed for each slice in the phase control rescanning irradiation proposed in Non-patent Document 1. Also, to achieve the dose determined by therapy planning with high accuracy, the beam intensity needs to be adjusted with high accuracy.
However, the beam intensity is adjusted by control of an upstream device such as an accelerator. It takes time to adjust the beam intensity, and once the beam intensity is changed, it is difficult to ensure reproducibility of beam properties (actual beam intensity, beam position, or beam size) when the beam is introduced into the irradiation apparatus. It can be supposed that the beam properties are checked and the result is adjusted by control of the upstream device, but this is not realistic because of an increased therapy time.
In the phase control rescanning irradiation, the respiration waveform signal of the patient is previously obtained, and the gate width of one respiration is set based on an amplitude and a cycle of the previously obtained respiration waveform. Then, based on the set gate width, proper beam intensity is calculated for each slice to design an irradiation pattern (therapy planning). If, therefore, the previously obtained respiration waveform does not match a respiration waveform when the patient lies down on a therapy bed in therapy irradiation, uniformity of dose distribution may be lost in one planned slice surface. Particularly, in actual therapy, the respiration waveform may change during therapy irradiation, and therefore non-uniformity of dose distribution in such a case may become a serious problem.
(Configuration and Operation of Conventional Apparatus)
In view of the above-described points, a specific configuration and operation of a conventional apparatus will be described.
FIG. 1 shows an exemplary configuration of a conventional particle beam irradiation apparatus 300 using a respiration synchronized irradiation method. The particle beam irradiation apparatus 300 includes a beam generation unit 10, a beam emission control unit 20, a beam scanning unit 30, a beam scanning instruction unit 40, a dose monitor unit 50, a position monitor unit 51, a ridge filter 60, a range shifter 70, a control unit 80, a respiration measurement unit 81, a respiration gate generation unit 82, or the like.
The particle beam irradiation apparatus 300 is an apparatus that irradiates an affected region 200 of a cancer patient 100 with a particle beam obtained by accelerating particles of carbon or the like or protons to high speeds for cancer therapy. The particle beam irradiation apparatus 300 can discretize the affected region 200 into three-dimensional lattice points, and perform a three-dimensional scanning irradiation method of sequentially scanning the lattice points with a particle beam with a small diameter.
Specifically, the affected region 200 is divided in a particle beam axis direction (a Z-axis direction in a coordinate system on the upper right in FIG. 1) by flat plate units referred to as slices, two-dimensional lattice points (lattice points in X-axis and Y-axis directions in a coordinate system on the upper right in FIG. 1) of divided slices such as a slice N, a slice N+1, and a slice N+2 are sequentially scanned for three-dimensional scanning.
The beam generation unit 10 generates particles of carbon ions or protons, and an accelerator such as a synchrotron accelerates the particles to such energy that can reach deep into the affected region 200 to generate a particle beam 90.
The beam emission control unit 20 performs on/off control of emission of the generated particle beam 90 based on a control signal output from the control unit 80.
The beam scanning unit 30 deflects the particle beam 90 in an X direction and a Y direction and two-dimensionally scans a slice surface, and includes an X electromagnet 30a for scanning in the X direction and a Y electromagnet 30b for scanning in the Y direction. To the X electromagnet 30a and the Y electromagnet 30b, a driving current of each electromagnet is applied from the beam scanning instruction unit 40 as an instruction signal to instruct a scanning position.
The range shifter 70 controls a position in the Z-axis direction of the affected region 200. The range shifter 70 is constituted by, for example, acrylic plates having different thicknesses. By combining the acrylic plates, energy of the particle beam passing through the range shifter 70 can be gradually changed. That is, an in-body range can be changed depending on the position in the Z-axis direction of the slice of the affected region 200. The in-body range by the range shifter 70 is generally controlled to be changed at regular space intervals, and the interval corresponds to an interval between lattice points in the Z-axis direction. The in-body range may be switched, as mentioned above, by inserting an object for attenuation such as the range shifter 70 on a path of the particle beam, or instead, may be switched by changing energy itself of the particle beam by controlling the upstream device.
The ridge filter 60 is provided to diffuse sharp peaks referred to as Bragg peaks of the dose in an in-body depth direction. A diffusion width of the Bragg peak by the ridge filter 60 is set to be equal to the thickness of the slice, that is, the interval between the lattice points in the Z-axis direction. The ridge filter 60 for three-dimensional scanning irradiation is constituted by a plurality of aluminum rod members each having a substantially isosceles triangular section. A difference in path length caused when the particle beam passes through the isosceles triangle allows the Bragg peaks to be diffused, and the shape of the isosceles triangle allows the diffusion width to be set to a desired value.
The dose monitor unit 50 monitors the dose of irradiation, and includes, in a casing thereof, an ionization chamber that collects charges generated by an ionization effect of the particle beam with a parallel electrode, a SEM (Secondary Electron Monitor) device that measures secondary electrons emitted from a secondary electron emission film placed in the casing, or the like.
The position monitor unit 51 identifies whether the particle beam scanned by the beam scanning unit 30 is located in a proper position, includes a configuration similar to that of the dose monitor unit 50, and uses a charge collecting electrode that is, for example, divided into strips or includes a plurality of wires.
The control unit 80 controls the entire particle beam irradiation apparatus 1, performs on/off control of beam emission to the beam emission control unit 20, sends instructions on beam scanning to the beam scanning instruction unit 40, and performs control of a range shift amount with slice changes of the range shifter 70, and so on.
The beam scanning instruction unit 40 determines scanning positions in the X direction and the Y direction or scanning timing of each slice based on the instructions from the control unit 80, and outputs a driving current of the X electromagnet 30a or the Y electromagnet 30b to the beam scanning unit 30.
The respiration measurement unit 81 and the respiration gate generation unit 82 are both used for operation using a respiration synchronized irradiation method, and functions thereof will be described later.
FIG. 2 is a flowchart showing an example of a basic process (process without respiration synchronization) of three-dimensional scanning irradiation by the conventional apparatus.
First, an affected region is virtually divided into a plurality of slices along a beam axis, and one of the divided slices is selected. For example, a slice in the deepest position in the affected region is first selected. Also, incident energy of a particle beam and a combination of acrylic plates in the range shifter 70 are selected and set depending on the position of the selected slice (Step ST1).
Then, the number M of lattice points to be irradiated with the particle beam and a lattice point position (Xi, Yi) [i=1 to M] that is a spot to be irradiated are selected depending on the shape of the affected region in the deepest slice, and the beam scanning unit 30 sets a direction of the particle beam to the lattice point position (Xi, Yi) on the slice (Step ST2). Then, emission of the particle beam is started (Step ST3). Energy distribution of the particle beam output from the beam scanning unit 30 is expanded in the Z-axis direction by the ridge filter 60 so that an in-body range distribution width corresponds to the slice width.
The irradiation dose for the lattice point (Xi, Yi) is monitored by the dose monitor 4, and when the irradiation dose for the target lattice point reaches a planned dose, a dose completion signal is output to the control unit 80, and the control unit 80 receives the signal (Step ST4).
The three-dimensional scanning irradiation method is mainly classified into a spot scanning method and a raster scanning method. The spot scanning method is a method of stopping beam emission while the position of the particle beam is moved from a certain lattice point to a next lattice point, and restarting the beam emission after the completion of the movement. Thus, the beam emission is intermittently performed during a scan of the same slice.
On the other hand, the raster scanning method is a method of continuing beam emission without stopping even while the position of the particle beam is moved from a certain lattice point to a next lattice point. Specifically, the beam emission is continuously performed during a scan of the same slice.
In both the spot scanning method and the raster scanning method, the position of the particle beam is kept at a lattice point until a planned dose at each lattice point is reached, and moved to a next lattice point after the planned dose is reached.
In Step ST5, it is determined which of the spot scanning method and the raster scanning method is used. When the spot scanning method is used, the beam emission is once stopped (Step ST6), and the beam position is moved to a next spot. This process is repeated to a final spot of the target slice (Step ST7).
Meanwhile, when the spot scanning method is not used, that is, when the raster scanning method is used, the beam emission is continued to the final spot without stopping the beam emission.
When irradiation of one slice is finished (YES in Step ST7), the beam emission is once stopped in both the spot scanning method and the raster scanning method, the process returns to Step ST1, and a next slice is selected and setting of the range shifter 70 is changed. The above-described processes are repeated until a final slice is reached (Step ST9).
Specifications required for the above-described irradiation procedure are described, for example, in a data file referred to as an irradiation pattern file, and the data file is transferred to the control unit 80 before start of therapy irradiation. In the irradiation pattern file, a range shifter thickness that provides a slice position, a driving current value of the X electromagnet 30a and the Y electromagnet 30b that provide a beam position corresponding to the lattice point (X, Y), an irradiation dose for each lattice point, or the like are described for each lattice point in order of irradiation.
FIG. 3 shows an example of a conventional scanning pattern on a slice. A path pattern from a start lattice point A to a final lattice point B is previously determined by therapy planning, and a particle beam is sequentially scanned in one direction along the path pattern.
Next, a conventionally proposed respiration synchronized irradiation method will be described. In the case that an affected region 200 is in an organ such as lung or liver that is treated by the respiration synchronized irradiation method, for example, an LED (not shown) or the like is attached to chest of the patient (not shown in FIG. 1). An image of a movement of the LED is obtained by a respiration measurement unit 81 constituted by a video camera or the like, and the movement of the LED is further made into one-dimensional data to obtain a respiration waveform signal. The respiration waveform signal is sent to the respiration gate generation unit 82, and a respiration gate is generated only during a period when the respiration waveform is lower than a predetermined threshold. The respiration gate is sent to the beam emission control unit 20, the beam is emitted only during a period when the respiration gate is on, and the beam emission is stopped during a period when the respiration gate is off.
FIGS. 4A to 4C show a relationship between a respiration waveform signal (signal corresponding to a position change of an affected region by respiration), a threshold, a respiration gate, and beam emission. For spot scanning irradiation, beam emission and beam stop are further switched in movement of the spot position (this time interval is much shorter than a respiration synchronized gate signal), which is herein omitted (hereinafter the same).
Next, the respiration synchronized irradiation method (phase control rescanning irradiation method) in three-dimensional scanning irradiation proposed in Non-patent Document 1 will be described with reference to timing charts in FIGS. 5A to 5G. In this irradiation method, irradiation in one slice surface is repeated m times for one respiration gate to perform irradiation. In this case, beam intensity is adjusted so that m time irradiation in the slice surface just matches a time of one respiration gate width Gw. FIGS. 5A to 5G illustrate a case where four (m=4) time repeated irradiation is performed for one respiration gate.
First, a combination of acrylic plates of the range shifter 70 is set so that a beam range matches a next slice position. Simultaneously, the beam generation unit 10 constituted by an accelerator or the like adjusts the beam intensity. When the beam emission control unit 20 receives a setting completion signal of the range shifter 70 from the control unit 80, and receives a beam intensity setting completion signal from the beam generation unit 10, the beam emission control unit 20 enters a beam emission ready state. Then, beam emission starts in synchronism with a start of a respiration gate and beam emission. Thereafter, the beam scanning unit 30 performs beam scanning every time a dose completion signal from the dose monitor 50 is received, and irradiation and scanning of the lattice points in the slice surface are sequentially performed. In the spot scanning method, the beam emission is stopped when the particle beam is moved from a certain lattice point to a next lattice point, while in the raster scanning method, the beam emission is not stopped even during movement between the lattice points. Irradiation is performed up to a final lattice point and then a first scan for the slice is finished. This is automatically repeated m times for the slice, and when m time irradiation for all the lattice points is finished, the control unit 80 sends a one slice surface irradiation completion signal to the beam emission control unit 20 to stop the beam emission. Timing for providing the one slice surface irradiation completion signal is set a short time before the respiration gate is closed (see FIGS. 5B, 5C and 5D).
Then, the range shifter 70 is reset so that the beam range matches a next slice position. Simultaneously, beam intensity of the beam generation unit 10 is readjusted. When the beam emission control unit 20 receives a setting completion signal of the range shifter 70 from the control unit 80, and receives a beam intensity setting completion signal from the beam generation unit 10, the beam emission control unit 20 reenters the beam emission ready state, and starts a next respiration gate and starts beam emission for a new slice.
In this irradiation method, beam intensity for each slice is determined as described below. First, a respiration waveform of a patient is previously obtained, and a respiration gate width Gw is calculated from the waveform and a threshold. Then, beam intensity for each slice is determined so that a total time when m repeated scans are performed in each slice surface substantially matches the respiration gate width Gw. This is determined in the stage of therapy planning.
As described above, generally, the size of an affected region of a patient is not uniform in a beam axis direction. Specifically, as shown in FIGS. 6A to 6E, each slice has a different area, and the number of lattice points in the slice increases with increasing area of the slice. Meanwhile, the gate width of one respiration does not significantly change though there are some changes. Thus, a scanning time for one slice of the m divided parts of the gate width of one respiration needs to be the same for every slice (see FIG. 6D). This means that an irradiation time for one lattice point is reduced for a slice with many lattice points (slice with a large area) like the slice N+2 shown in FIG. 6A, while an irradiation time for one lattice point is increased for a slice with a few lattice points (slice with a small area) like the slice N.
On the other hand, the dose for each slice is determined by a product of an irradiation time and beam intensity at the lattice points in the slice. Thus, for example, when the same dose is required for each slice in therapy planning, high beam intensity needs to be set for the slice with many lattice points (slice with a large area), while low beam intensity needs to be set for the slice with a few lattice points (slice with a small area) (see FIG. 6E). Specifically, the beam intensity needs to be changed for each slice in the phase control rescanning irradiation proposed in Non-patent Document 1. Thus, the respiration synchronized irradiation method proposed by the Non-patent Document 1 has problems described below.
First, the beam generation unit 10 needs to adjust beam intensity for each slice. When a beam intensity determined before start of therapy is newly set to the apparatus, beam properties such as actual beam intensity, beam position (axial deviation), or beam size are checked in a usual procedure. However, checking these beam properties every time the beam intensity is changed extremely increases a therapy time, which is unrealistic. Thus, therapy irradiation with insufficient check of the beam properties may be performed. This prevents uniformity of dose distribution on the slice surface from being ensured.
Also, since the respiration gate width is set based on the previously obtained respiration waveform, and the beam intensity is determined based on the set respiration gate width, uniform dose distribution cannot be obtained when the respiration waveform does not match the respiration waveform of the patient immediately before start of the therapy and when the respiration waveform changes during the therapy.
Further, therapy planning is performed using the beam intensity as a parameter, which may provide a complicated calculation method of the therapy planning. Complicated therapy planning prevents an optimum solution from being obtained.
Also, the beam intensity needs to be adjusted, which requires, for example, a large dynamic range of the dose monitor 50 or the like and increases costs. Particularly, for a slice with a short irradiation time, irradiation is performed with reduced beam intensity, which may reduce the S/N ratio of the monitor.